Many automotive vehicles include heat exchangers such as condensers, evaporators, heater cores and coolers generally made of aluminum or aluminum alloys. These heat exchangers are alternating rows of tubes or plates. The heat exchangers often include convoluted fins brazed to the external surfaces of the tubes and turbulators disposed within the tubes and brazed to their inner surfaces. One way of brazing the fins and turbulators to the tube surfaces is by a vacuum furnace. Also, a process known as "controlled atmosphere brazing" (CAB) has been employed.
CAB furnace brazing typically is preferred over vacuum furnace brazing due to improved production yields, lower furnace maintenance requirements and greater braze process robustness. When aluminum components are exposed to air, the surface layer oxidizes and forms aluminum oxide. Although heat exchangers are pre-cleaned using alkaline cleaning agents which reduce the native aluminum oxide layer, the surface of the heat exchanger will re-oxidize in the CAB furnace due to the presence of the oxygen and water vapor in the nitrogen gas used in the furnace. In order to braze aluminum components together, a flux is provided at a joint between the tube and any component to be joined thereto in order to disrupt any aluminum oxide which might interfere with the formation of a sound joint. A flux commonly used in CAB furnace brazing is NOCOLOK.TM. (potassium fluoaluminate represented often as "KALF").
Magnesium is commonly included in aluminum based tubing or core materials to improve their strength and corrosion resistance. Magnesium is also generally included in the aluminum alloy cladding generally disposed on the core materials. U.S. Pat. No. 5,422,191, issued Jun. 6, 1995, to Childree discloses aluminum cladding materials which include lithium in addition to magnesium to increase the post braze strength of the brazed joint. Childree teaches that for CAB processing, NOCOLOK.TM. flux can be used.
U.S. Pat. No. 5,771,962, issued Jun. 30, 1998, to Evans et al. which is incorporated herein by reference discloses that the use of a standard KALF flux works less than desirable with core and clad materials which contain desirably high levels of magnesium. Evans et al. teaches a modified aluminum brazing flux including lithium fluoride, cesium fluoride or their mixture into an aluminum flux like NOCOLOK.TM.. Evans teaches that because the lithium and cesium in the flux have relatively low melting temperatures compared to magnesium, the lithium and cesium will melt first and flow into the joint area before the magnesium forming a sound braze joint.
As disclosed in Evans et al., the magnesium melts during processing and flows into the joint area. At high processing temperatures, magnesium readily forms magnesium oxides which are not broken down by conventional aluminum fluxes such as KALF and hence this oxide and the aluminum oxides present on the aluminum surfaces interfere with the integrity of the brazed joint. Such interferences occur by reducing the "wetability" of the molten clad layer and its ability to form an effective braze joint. Additionally, because a conventional KALF flux is not effective in CAB brazing for disrupting the complex MgO and Al.sub.2 O.sub.3 surface oxide, if and when wetting does occur, the braze joint is discontinuous and does not represent a sound braze joint. The end result of using a conventional KALF flux is a heat exchanger with porous and weak braze joints.
The heat exchanger assembly disclosed in Evans et al. has an aluminum based alloy cladding disposed on the core materials. The cladding may include a weight percentage of lithium (Li) along with other elements. The inclusion of the lithium in the clad material acts to lessen the magnesium from migrating out of the core material, creating a barrier to lessen the formation of undesirable magnesium oxides which interfere with the formation of a sound braze joint.
Although the cladding disclosed in Evans et al. provides a magnesium barrier, it is desirable to reduce the amount of lithium used. The less lithium needed to establish a sufficient magnesium barrier, the less aluminum is required, thereby reducing the weight of the cladding and, in turn, reducing costs.
Thus, what is needed is a more efficient magnesium barrier within a heat exchanger assembly.
Although the core material used in Evans et al. is strong, it is desirable to use the strongest core material as is economically feasible. The stronger the core material used, the lighter the heat exchange assembly will be, thereby reducing manufacturing cost and increasing fuel efficiency.
Thus, what is needed is a stronger core material of a heat exchanger assembly to lessen the weight thereof.